Low Flow Cutoff Calculator (Square-Root DP Flow)
By Santaji GadepatilinUpdated October 29, 2026
With square-root extraction, flow = √DP, so tiny DP noise near zero becomes a large, jumping flow reading. A low-flow cutoff forces the output to zero below a set point. This low flow cutoff calculator converts the cutoff between flow and DP and shows how much the root amplifies noise — see square-root extraction in DP flow measurement.
How to use the low flow cutoff calculator
Enter the cutoff as a percentage of maximum flow, the maximum flow and the DP at maximum flow (the transmitter’s calibrated span). You get the cutoff in DP terms, the actual cutoff flow and DP, and the noise gain at that point. To find the flow corresponding to a DP cutoff set in a transmitter, clear the flow cutoff and enter the DP percentage.
Low flow cutoff formulas
Flow fraction q = √(DP fraction) ⇔ DP fraction = q²
Noise gain = d(q) ÷ d(DP) = 1 ÷ (2q)
Example
Cutoff at 5% flow on a 0–100 m³/h meter with 250 mbar DP span:
- DP at cutoff = 0.05² = 0.25% of span → 0.625 mbar
- Cutoff flow = 5 m³/h
- Noise gain = 1 ÷ (2 × 0.05) = 10 — each 0.1% of DP noise shows as 1% flow
Flow, DP and noise gain
| Flow (% of max) | DP (% of max) | Noise gain |
|---|---|---|
| 1% | 0.01% | 50 |
| 2% | 0.04% | 25 |
| 3% | 0.09% | 16.6667 |
| 5% | 0.25% | 10 |
| 7% | 0.49% | 7.14286 |
| 10% | 1% | 5 |
| 20% | 4% | 2.5 |
| 50% | 25% | 1 |
| 100% | 100% | 0.5 |
Where to apply the cutoff
- In the transmitter (many smart DP transmitters have a configurable low-flow cutoff and linear-to-root transition).
- Or in the DCS/PLC flow block — but only in one place, not both.
- Fix zero drift first: re-zero the transmitter at line pressure with equalising valve open before blaming noise.
Frequently asked questions
Why is a low-flow cutoff needed with square-root extraction?
Near zero the slope of √DP becomes very steep. At 1% flow, a DP change of 0.01% of span moves the flow reading by 0.5% — so transmitter noise and zero drift look like real flow.
What cutoff value is typical?
Most DP flow transmitters and DCS blocks use a cutoff between 5% and 10% of flow, which is only 0.25% to 1% of DP.
What is the noise gain?
It is the slope of the square-root curve, d(flow) ÷ d(DP) = 1 ÷ (2 × flow fraction). At 5% flow it is 10, so 0.1% DP noise becomes 1% flow noise.
Does the cutoff hide real flow?
Yes, anything below the cutoff reads zero. Totalizers will miss it, so choose a meter with better turndown if low flows matter.